The invention relates to a communication system for the transmission on a telephony channel of speech signals located within a speech band as well as at least one data channel comprising data signals modulated on a carrier located within a data band which is outside the speech band, the system comprising at least three stations, each station including a transmitter and a receiver which each operate at a mutually different frequency--belonging to a frequency pair--for half-duplex data communication combined with full-duplex speech communication.
Such a communication system is generally known and is, for example, suitable for use for service purposes in a satellite communication system.
CCITT recommendation H 34 describes how for combined telephone and telegraph communication the telephone channel is divided into a speech band and a plurality of data channels forming a data band.
In addition, it is known that a communication system for combined data and speech communication operates on one frequency pair. In such a system a station transmits on a first frequency and receives on a second frequency, whereas a second station receives on the first frequency and transmits on the second frequency. The communication between two stations belonging to the group is either done through wires or may be done by means of radio communication; in the latter case, for example by way of a satellite. The communication through the data band is restricted to half-duplex, when it is assumed that, in order to be able to receive the data signals transmitted by a station at all the other stations, the frequency of the receivers in all the other stations must be equal to the frequency of the transmitted signal. Thus, it is not possible for a second station to operate simultaneously in the same mode on the second frequency as the receivers of the remaining stations are tuned to the first frequency. Full-duplex communication over the speech band is only possible between two stations which belong to the group. The other stations receive only the speech signal from one of the two stations.
A special problem is encountered in the combined communication of speech and data. If, namely, a first station transmits on the first frequency and receives on the second frequency, whereas a second station transmits on the second frequency and receives on the first frequency, all the other stations also receive on the first frequency. However, when the first station stops the supply of data before the speech connection is broken, the data supply of the second station cannot be transmitted because the other stations, since they receive on the first frequency, would not receive these data. The consequent waiting times may be considerable.